Soils - sodic and acidic

The effects of soil salinity have been well publicised, with dramatic images of bare and badly gullied land glistening with salt crystals, and of dead and dying trees as a stark reminder of the serious impacts of salinity. There is, however, another form of soil degradation that affects an even greater area of land in Australia than salinity, but which has received very little publicity. It is known as soil sodicity.

Sodic soils occupy almost 1/3 of the land area of this country, causing poor water infiltration, surface crusting, erosion and waterlogging. As in the case of salinity, some soil sodicity occurs naturally, although farming practices have certainly also made a major contribution.

Sodic soils are soils that contain a large amount of sodium ions (sodium atoms with a positive charge) attached to clay particles. This happens in circumstances where soluble sodium salts, such as sodium chloride, sulphate and carbonate, have broken down, so that the chlorine, sulphate and carbonate have leached away, leaving behind sodium.

When there is excess sodium attached to clay particles, the soil swells and the clay particles disperse when in contact with water, rather than sticking together, causing the soil structure to slump and collapse. Tiny particles of dispersed clay then block soil pores and cracks. When soil is affected by sodicity, the surface often becomes hard-setting, and forms a dense impermeable layer that is susceptible to waterlogging, reduced aeration and erosion.

Soils affected by sodicity are often alkaline (pH above 8.5), which further restricts the growth of plants.

Agricultural practices exacerbate these processes through the clearance of vegetation, cultivation, and overgrazing.

Sodicity has serious impacts on farm production, as well as significant off-site consequences.

When soil becomes badly affected by sodicity, it becomes too waterlogged, or too dry for optimal plant growth. The collapse of soil structure reduces the amount of oxygen available for plant growth and interferes with the ability of the roots of plants to penetrate the hard-setting soil. Also, the high alkalinity of the soil causes some nutrients to become unavailable, or too toxic for plant growth. In addition, sodic soils are highly dispersible, and are prone to gully erosion and tunnelling of earthworks.

Soil sodicity also has significant impacts off-the-farm. Accelerated soil loss and run-off result in increased sedimentation in streams and rivers. Organic matter, nutrients and pesticides, absorbed by clay suspended in runoff from sodic soils, are carried into waterways. This can lead to a decline in the quality of surface water as a result of increased turbidity and eutrophication, and in increased siltation downstream. Aquatic flora and fauna can be affected by a general reduction in available oxygen in waterways, nutrient imbalance and an increase in the siltation of habitat. There are also impacts on human settlements, where deterioration of water quality presents a direct cost to water users.

The National Land and Water Resources Audit estimated that sodic soils occupied around 109,219,000 ha of land in Australia in 2000. This compares with just 3,206,000 ha of soils affected by salinity. The cost of soil sodicity to agricultural production alone is substantial - estimated at $1.5 to 2 billion annually. The total cost of the off-farm impacts of sodic soils has not yet been estimated, but is certainly significant.

There are at present no national or state strategies in place targeting soil sodicity. Management needs to occur at the farm level.

Sodic soils can be directly treated through the application of gypsum, which serves to replace the excess sodium in sodic soils with calcium. If the soil is particularly alkaline, gypsum is less effective, and it may be necessary to plant acidifying legumes to reduce pH. The application of lime to sodic soils acts in a similar manner to gypsum, but is slower acting and less effective.

Although the application of gypsum can effectively counter sodicity in the short run, longer term management strategies need to be in place to maintain and increase organic matter in soils. Increased organic matter can improve hard-setting soils, and it can also enhance the effect of gypsum. Sodicity can also be reduced by maintaining adequate vegetation cover, leaf litter or stubble on the soil surface.

Many soils in Australia are naturally acid, due partly to the nature of their parent materials and partly to the great age of the continent, involving exposure of soils to acidifying processes over a very long period of time. But in many areas, these natural processes of acidification are being accelerated as a result of agricultural activities. This is causing losses in crop and pasture production, as well as significant social, environmental and financial costs associated with the off-site effects of acid soils.

The effects of acid soils are not usually noticed in crops and pastures until the soil becomes highly acidic. It is then difficult and expensive to control and manage.

Soils between pH of 5.5 and 8.5 are generally suitable for the growth of most plants. Soils are considered to be acid if the pH is below 5.5.

Naturally acid soils occupy over one-third of the Australian landmass. These soils are derived from old and weathered parent materials that have lost most of their basic minerals except for sand and metal oxides. There are extensive areas of moderately acid soils in regions where the rainfall exceeds the rate of evaporation, resulting in the leaching of basic minerals from the root zone.

It has been estimated that over 60 million hectares of naturally acid soils may have existed in Australia prior to European settlement, mainly in the areas of higher rainfall that are now used for agriculture. It is thought that a further 30 million hectares of acid soils have developed since the onset of clearing for agriculture. Some estimates are more extreme, suggesting that around 35 million hectares of agricultural land in areas with annual rainfall above 600mm have become highly acid (with a pH value lower than 5.0), and a further 55 million hectares of moderately acid topsoils (between pH 5.0 and 6.5) on agricultural lands with 300-600 mm of rainfall.

Four main causes of increased soil acidity through farming are recognised: (1) Removal of plant and animal products (2) Leaching of nitrogen (3) The build-up of organic matter (4) The use of nitrogen fertilisers.

Removal of plant and animal products

The removal of plant and animal produce that contains calcium, potassium and other basic minerals can result in an increase in soil acidity. The degree of acidification will depend on how alkaline is the product removed and on how much is removed. In situations where little actual product is removed, as for example in wool production, this effect is minimal. But other farming, such as the cutting of lucerne hay, can have a major acidifying effect.

Leaching of nitrogen

When there is more nitrate in the soil than the plants can use, the nitrate can drain, or leach, below the plant roots and so into the groundwater system, leaving more hydrogen ions in the soil and resulting in high levels of acidity. Nitrate leaching can result from the inappropriate use of nitrogen fertilisers and is especially common in intensive forms of farming like horticulture. Pastures based on annual species or involving heavy application of nitrogen fertilisers can increase the risk of nitrogen leaching..

The build-up of organic matter

While organic matter in soil is natural and beneficial, an excess of it can lead to acidity. This can come about as the result of long term and regular use of fertilisers on improved pastures and the promotion of growth of clover.

The use of nitrogen fertilisers

The degree of acidification caused by nitrogen fertiliser depends on the type of fertiliser. Ammonium sulphate and monoammonium phosphate (MAO) and are the most acidifying, followed by diammonium phosphate (DAP). Ammonium nitrate, anhydrous ammonia and urea are less acidifying. Sodium and potassium nitrate are not acidifying.

Superphosphate does not have a direct acidifying effect. However, it stimulates the growth of clovers and other legumes which fix nitrogen, thus increasing the amount of nitrogen in the soil. This increase the likelihood of nitrogen leaching and consequent acidification.

There are many problems associated with acid soils on agricultural lands, the most significant being considerable losses in farm productivity. This can occur when the soil pH value drops to 5.5, and below 5.0 other specific problems can arise. Soil acidity affects the availability of various nutrients, including calcium and molybdenum, and under highly acid conditions the processes by which plants take up nutrients are inhibited. Acid soils often show a poor response to superphosphates.

Soil acidity can also result in specific toxicity problems associated with the presence in the soil of heavy metals, which become more soluble with decreasing pH values. Under highly acid conditions these heavy metals can be released into the soil matrix, with toxic effects on plant growth and soil biota. The most serious element limiting the growth of plants in acid soils is aluminium. At medium levels, free aluminium decreases root growth and branching.. Free aluminium ions in the soil can also displace calcium and magnesium ions, and can reduce the ability of plants to access remaining nutrient minerals. Increased levels of aluminium as an outcome of acid conditions can lead to greater fixation of inorganic phosphate in forms unable to be taken up by plants. Manganese is another mineral that becomes more soluble with decreasing pH values. While manganese is an essential nutrient for plant growth, in acid soils it can build-up to highly toxic levels.

Such mineral toxicity results in lower crop yields, and it restricts the ability to grow crop species that are intolerant to such conditions. Soil organisms like bacteria, fungi, insects and other animals that play a significant role in soil metabolism, are also inhibited by the toxic effects of acid soils. Over time, the serious effects that acid conditions have on plant growth and soil organisms results in an increased risk of erosion.

The impacts of acid soils often extend beyond the place where the problem arises. The erosion and leaching of excess nutrients from acid soils can lead to water pollution downstream, like increased salinity, algal blooms and acidity downstream. These off-site impacts of acid soils are often felt most by urban and industrial users of waterways lower in the catchment. The reduction of pH and the concentration of toxic elements in acid-affected waterways can also cause dramatic fish kills and harm to other stream organisms.

The cost of treating the soil acidity is high, but failure to take action often results in further loss of income. The total annual on-farm cost of acid soils in Australia is estimated to be around $630 million.

The off-site costs of acid soils are more difficult to quantify and so no national estimate has yet been made. Estimates need to allow for the damage to infrastructure, including fences, earthworks, road and building foundations, houses and power and telephone poles and lines.

The cost of acidifying rivers is also not known, but there is a growing concern about the quality of inland waters. There is also an increased awareness of the potential damage to the Great Barrier Reef and other coastal and marine ecosystems resulting from the cultivation of naturally acid-sulphate soils on the coast. When these soils are disturbed and exposed to the air, mostly for agricultural purposes, the sulphur compounds release toxic fumes of sulphuric acid, causing damage to coastal and marine ecosystems. The resulting loss of amenity could have severe consequences for the burgeoning tourism industries in these areas.

At present, the management of acid soils is largely confined to the on-ground treatment of the problem on individual properties. Acid soils can be effectively neutralised through the application of lime. Lime acts to increase the pH value of surface soils. It can be applied directly to the soil surface or, for faster results, to the sub-soil. While the application of lime can successfully address acid problems on agricultural lands, huge quantities need to be applied for even a modest effect. The cost of lime treatment is very high, and given the extensive areas of land affected, it is not a practical method of management in the long term.

In view of the significant off-site impacts of acid soils, a catchment-based approach will need to be developed to ensure long-term, ecologically sustainable management of the problem, involving farming techniques that do not result in increased soil acidity.

Born and raised in Canberra, Alice Thompson was brought up with an appreciation of, and interest in the environment, leading her to study at the Australian National University, majoring in Geography/Human Ecology and Population Studies, and her involvement in the Nature and Society Forum (NSF). She now lives in Sydney where she currently pursues a career in Government working for the NSW Office of the Australian Bureau of Statistics (ABS). Before joining ABS Alice was employed by NSF as a Research Officer to prepare reports on important ecological issues in Australia.